Update README.md

Author: seth-reeser

README.md

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 Platform | System | Release Cycle | Latest Release | [Vagrant Box](https://www.vagrantup.com/docs/boxes.html)
 ---------|--------|---------------|----------------|---------------------------------------------------------
-Windows | Windows Server 2012 R2 Standard 64-bit | Monthly | 2017 June | [`devopsgroup-io/windows_server-2012r2-standard-amd64-nocm`](https://atlas.hashicorp.com/devopsgroup-io/boxes/windows_server-2012r2-standard-amd64-nocm)
+Windows | Windows Server 2012 R2 Standard 64-bit | Monthly | 2017 September | [`devopsgroup-io/windows_server-2012r2-standard-amd64-nocm`](https://atlas.hashicorp.com/devopsgroup-io/boxes/windows_server-2012r2-standard-amd64-nocm)
 
 
 ## Vendor Update Release Cycle